The present invention relates to methods of using kinetic gas hydrate inhibitors in subterranean operations.
Gas hydrates are a growing concern in oil or gas production at least in part because gas hydrates can present flow assurance problems in onshore wells, offshore wells, and pipelines. Gas hydrates are a common form of a unique class of chemical compounds known as clathrates, in which a rigid, open network of bonded host molecules enclose, without direct chemical bonding, appropriately sized guest molecules of another substance. In the case of gas hydrates, water acts as the host molecule, enclosing gas molecules such as methane, thereby yielding ice-like crystals of gas and water.
Gas hydrates normally are found in cold climates, in deepwater environments, or at any point in a gas system where the gas experiences rapid expansion. As this lattice expands and gains mass, it can block tubings, flow lines, pipelines, or any conduit through which produced gas flows such as a drill string or a blow out preventer.
As deepwater drilling and production increases, the problems associated with hydrate formation may increase. Deepwater is an ideal breeding ground for the growth of gas hydrates, and when these ice-like crystals form in the circulating system, attempts to manage them can be costly and dangerous. For the same reason, as operators search for hydrocarbons in colder regions such as Siberia, Alaska, and Canada, hydrates increasingly will become a cause of significant production problems.
Operators can take precautionary measures by reducing the water available for gas hydrate formation. For example, after a pipeline for the transportation of light hydrocarbons such as natural gas has been repaired, constructed, hydro-tested, or otherwise exposed to water, it is mandatory that water remaining in the pipeline be removed. Light hydrocarbon gases are particularly susceptible to forming hydrates with water, which can and often do reduce or block the flow of gases through pipelines.
To solve gas hydrate problems, the industry traditionally uses thermodynamic chemistry to dissolve and inhibit hydrate formation. Gas hydrates offer two distinct problems for the scientists and engineers who design systems to mitigate the hydrate effect. The first problem concerns dissolution. When a hydrate plug forms, it must be melted to unblock the transmission conduit. For example, if a hydrate plug forms at the mudline in a deepwater completion, the operator must find a way to melt the ice plug in situ before production can proceed.
The second problem concerns inhibition. The goal is to prevent hydrate formation in the first place. However, to inhibit hydrate formation, the inhibitor must be present before a system reaches hydrate-forming conditions (e.g., low-temperature, high-pressure flow regimes). The traditional chemical approach to hydrate inhibition and dissolution has been to add sufficient quantities of a thermodynamic inhibitor to the production system. “Thermodynamic inhibition” refers to the chemicals' abilities to suppress the point at which hydrates will form. A thermodynamic inhibitor lowers the temperature at which hydrates form (at a constant pressure), but it may also increase the pressure at which hydrates form (at a constant temperature). By shifting the hydrate equilibrium toward higher pressure and lower temperature conditions, inhibitor chemicals make the water/gas system more resistant to hydrate formation.
However, mitigating the formation of gas hydrates with thermodynamic inhibitors requires significant quantities of the thermodynamic inhibitor. Methanol and glycols, usually ethylene glycol or triethylene glycol, are traditionally used as thermodynamic inhibitors. Because glycols can significantly increase the cost of a subterranean operation, their use is usually limited to facilities that include a glycol recovery or regeneration system. Further, because of the quantities of thermodynamic inhibitors that need be present, the compositions and concentrations of other additives in treatment fluids may be limited.
Thus, there is a need for improved methods of inhibiting gas hydrate formation in situ that requires less hydrate inhibitor and provides for more variability in the treatment fluid composition.